Habitual sleep duration varies greatly among individuals. The biological meaning of this variation is unknown, and so are the triggers for sleep and awakening. We proposed that the differences in sleep duration are not just a consequence of exogenous (e.g. social) factors, but result from differences in the endogenous circadian pacemaker. The circadian pacemaker, which is located in the suprachiasmatic nucleus of the hypothalamus, controls the timing of sleep and wakefulness, and regulates the circadian rhythms of a variety of physiological variables. We hypothesized that the 'internal biological night', as indicated by the nocturnal interval of active melatonin secretion, low body temperature, and increasing sleepiness is shorter in short sleepers than in long sleepers. Young (21-31 years) healthy male and female short sleepers (N=7; sleep duration <6 hours) and long sleepers (N=5; >9 hours) were selected on the basis of questionnaires, 2-4 week sleep logs and wrist motor activity recordings. Subjects underwent a ~40-hour constant routine protocol during which they stayed awake in bed in dim light, and without time cues. Fluids and isocaloric meals were given every hour and every 2 hours, respectively. The purpose of this protocol was to minimize or distribute evenly the masking influences of sleep, posture, exercise, meals, and light, which distort the endogenous circadian rhythms. Blood samples were taken every 30 minutes for 24 hours. Plasma melatonin was measured by radioimmunoassay. Body temperature was recorded continuously with a rectal probe. Sleepiness rating were obtained every 30 minutes on 100-mm visual analog scales. The duration of the nocturnal interval of detectable melatonin levels did not differ between groups. In contrast, the duration of the nocturnal trough in body temperature was shorter in the short sleepers than in the long sleepers (9.7+/-0.4 (SEM) vs. 11.8+/-1.1 hours; p<0.05, Wilcoxon), and so was the nocturnal interval of increasing sleepiness (8.9+/-0.3 vs. 10.5+/-0.8 hours; p<0.05). The peak in sleepiness coincided in both groups with habitual wake-up time, which occurs about 2.5 hours later in the long sleepers. The difference in the circadian rhythms of body temperature and sleepiness are consistent with our hypothesis. Previously, it has been shown that both body temperature and sleepiness are also directly affected by melatonin. Thus, if melatonin constitutes a circadian signal for sleep, the absence of differences in the duration of melatonin secretion may reflect differences in the sensitivity to this signal in short and long sleepers.